1. The effects of various ions and chemicals were tested on the resting or active membrane of the heart of the wood-louse Porcellio dilatatus.
2. The curve relating the resting membrane potential to log [K+]o was found to correspond with the theoretical curve expected from the Nernst equation at higher concentrations only. Excess K+ decreased both amplitude and rate of rise of the response while the rate of decline was increased. In K+-deficient solutions the duration of the plateau phase was at first increased, then depressed. The addition of K+ to a bathing medium deprived for several minutes of this ion caused a large increase in the membrane potential and in the response height. The way in which the membrane was seen to react was tentatively attributed to an electrogenic active pumping mechanism.
3. In Na+-deficient solutions, the rate of rise and the height of the response were reduced while the resting membrane potential was decreased.
4. Ca2+-deficient solutions depolarized the membrane and decreased both amplitude and duration of the response. Cessation of activity occurred in Ca2+-free solution. In excess calcium the membrane was hyperpolarized. The rhythm and the rate of rising were decreased and the plateau phase depressed.
5. TTX blocked the heart activity, probably by acting upon the heart ganglion. Mn2+ depressed especially the humped plateau (when present) of the spontaneous responses.
6. TEA, caffeine and procaine transformed spontaneous activity of weak amplitude into large and complex overshooting responses. In TEA solutions, several stable levels of polarization were observed. Contrary to what occurred in the normal solution, depolarizing current pulses could trigger large all-or-none action potentials when TEA was present.
7. The TEA-induced regenerative response was analysed with the help of an intracellular stimulating current when [Na+]o and [Ca2+]o were varied. Additional data were obtained by applying TTX, Mn2+ or GABA. From the results, both Ca2+ and Na+ were thought to be involved in the ionic currents underlying spike type activity.
8. The spike-generating effect of TEA has been attributed to its property of increasing the membrane resistance and of allowing the ionic conductances which generate the weakly active component of the normal response, the plateau, but not the initial upstroke, to be amplified regeneratively.
9. The large spikes elicited by TEA were found relatively less effective than weak sustained depolarization in inducing strong contractions.
10. The functional significance of the data was tentatively interpreted by comparison with the properties of the heart of Limulus, Crustacea and vertebrates.
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